1. Field of the Invention
The present invention relates to implantable prosthetic hearing enhancement devices which vibrate portions of the inner ear so as to stimulate the sensory apparatus which enables an individual to hear sound.
2. Discussion of Background
Over 22 million Americans, roughly one in every fifteen individuals, suffer from sensorineural hearing impairment or "nerve deafness." This condition affects approximately 80% of significantly hearing impaired patients, and unlike conductive hearing loss, cannot be surgically corrected. Current rehabilitation relies on conventional ear-canal hearing aids. Unfortunately, the use of those aids in nerve deafness involves frequent problems resulting from acoustic feedback or "squeal," poor sound quality, and inability to deal effectively with background noise. In normal auditory function, incoming sound causes vibration of the ear drumo. This vibration is carried by the chain of middle ear bones, or ossicles, to a spiral fluid filled structure known as the cochlea. Projecting into the cochlea are thousands of specialized cells, called hair cells, which connect to fibers of the auditory nerve. Vibration of the cochlear fluid results in deflection of microscopic fibers (stereocilia) on the surface of the hair cell. This stimulates the hair cells to initiate transmission of a neural signal via the auditory nerve to the brain. Damage to these hair cells can result from the aging process, noise exposure, head injury, infections, treatment with some medications, and hereditary factors, and is the most frequent cause of sensorineural hearing loss.
Currently, sensorineural hearing loss (SNHL) can be partially rehabilitated with "behind the ear" or "ear canal" type hearing aids. Conventional hearing aids amplify sound arriving at a microphone external to the ear and then send that high intensity sound from a small speaker in the car canal, through the air in the canal to the ear drum. Problems exist, however, in transmitting the amplified signal through the air in the canal and along the bones of the ear with the necessary intensity to overcome the sensorineural loss. Distortion of the sound and acoustic feedback to the microphone are the principal problems. Acoustic feedback at high hearing aid volumes requires that a tight fitting ear mold be used. This solution, however, often causes or aggravates infections in the ear canal and makes conventional aids for sensorineural hearing loss uncomfortable for long-term wear.
Research and development over the past two decades has identified implantable aids as a means for circumventing problems found in conventional (acoustic transmission) hearing aids. Implantable aids work on the basic principle that vibrational energy can be directly imparted to the middle or inner ear through non-acoustic transmission. These methods require the use of an implanted vibrator connected to some structure of the middle ear or inner car which when displaced can produce vibrations that reproduce those generated in normal hearing. Implantable vibrators in use today utilize either a piezoelectric ceramic bimorph or an electromagnetic-permanent magnet couple.
Piezoelectric bimorphs consist of a bonded pair of piezoelectric materials. Piezoelectric materials lengthen or shorten with axially applied current. In the bimorph, two bonded pieces of piezoelectric material are oppositely aligned, so that when current is applied, they will deflect maximally in one direction or the other dependent on the polarity of the current. As an implant, the bimorph can have one end anchored to the skull with the other attached by some means to the ossicular chain in the middle ear. In this way electrical energy from an amplified signal can be transduced into vibrational energy in the middle ear system by non-acoustic transmission. To a large extent, research utilizing this approach has been conducted, see, for example, Gyo, K., Goode, R. L., Miller, C.: "Stapes Vibration Produced by the Output Transducer of an Implantable Hearing Aid", Arch. Otolaryngol. Head and Neck Surgery, Vol. 113, pp. 1078-1081 (1987), and Yanagihara et al, "Implantable Hearing Aid Using an Ossicular Vibrator Composed of a Piezoelectric Ceramic Bimorph: Application to Four Patients" American Journal of Otolaryngol., Vol 8, pp. 213-219 (1987). These groups have produced good results, but there are inherent problems with this approach. The piezoelectric implant totally disrupts the normal middle ear mechanism due to its attachment to the ossicular chain. These bones are no longer free to vibrate in response to incoming acoustic energy. Additionally, a means is required to transport the electric signal from the hearing aid's external pickup microphone and amplifier to the implanted device. Yanagihara et al. (1987) have attempted to solve this problem by using electromagnetic induction across the skin. This method, while somewhat effective also introduces additional transduction and amplification steps, with degradation of performance.
Hough et al., in "A Middle Ear Implantable Hearing Device for Controlled Amplification of Sound in the Human: A Preliminary Report" Laryngoscope, Vol 97, pp. 141-151 (1987), describe results obtained using magnet placements on the ossicles of the middle ear of animals and five implantation patients. The results reported in that study clearly indicate the potential benefits of this type of aid, but the degree of hearing enhancement achieved was less than that required for the rehabilitation of severe sensorineural hearing loss. Their approach relied on a magnet attached to an ossicle of the middle ear. That approach is restricted in its ability to deliver high amplitude vibrational energy to the inner car. Our pilot studies show that direct stimulation of the inner ear is achievable using a round window magnet. A device to significantly aid in the rehabilitation of moderate to severe sensorineural hearing loss must provide extremely high gain. Signal gain must also be accompanied by proper signal processing to achieve the flexibility and signal characteristics necessary for maximized sensorineural hearing loss rehabilitation.
In U.S. Pat. No. 3,764,748 there is disclosed an implantable hearing aid device which vibrates the cochlea using a bimorph crystal which imparts vibrations corresponding to sound waves entering the ear canal and vibrating the eardrum and/or ossicular bones. However, this technique has the disadvantage that artificial devices must be connected to the delicate structures of the normal acoustic input pathway. The device of the present invention avoids this drawback using an implementation which leaves the normal acoustic pathway and delicate structures associates therewith unobstructed by artificial devices, e.g., coils, microphones, etc.